Wednesday, January 16, 2013

Cookin' Wood

Curly Basswood Thermally Modified

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The correct technical term is 'Thermally Modified Wood'- its done in an oven, up to 400 degrees and smells like baking bread- cookin' wood. The Europeans are ahead of us on the process, I'm not surprised. We didn't invent the rocket either, but we were first to the moon once we kidnapped all those German rocket scientists. But I digress.

Interesting (and useful) changes take place in the structure of wood when subjected to temps of a few hundred degrees. Not as interesting as when you get above the ignition point, but still neat.

It all starts with a question: What makes wood interesting to fungus, bugs, and other decay inducing organisms? Sugar, of course, In different degrees, wood contains sugar inside the cells, used for metabolism. Syrup maple have a lot, I reckon, other woods not so much. We can deter those organisms by adding poisons- the buggers still bite, but they die. That's pressure treated lumber, and it has its own inherent problems- its toxic to us too, and treatment involves adding moisture to a KD product and the instabilities that go with that. Another issue is the species used in treating- the process works with softwoods, which do not wear as well as hardwoods.

More questions: Is there a better way? Can we remove the sugars, or otherwise render the wood inedible? Ahhh, now we're on an interesting track. Caramelized sugars are different enough to be uninteresting to the organism's we want to deter- however, sugars caramelize at temps above the normal course of kiln drying that we use to stabilize hardwoods. Hardwood lumber kilns often operate in the 180 degree F range. Wikipedia has a TMI article for people who want more:

Sugar caramelizes in the mid 300 degree F range, depending on the type of sugar.

So we have two issues to overcome. Kiln dried lumber still has some water inside the cell walls, and a little bit floating around outside too, typically 6% or more by weight. We need to take steps to get rid of that water gently- too much heat too fast and the expanding steam will likely damage (crack) the wood. Step one, heat above the boiling point, just a little bit, and let the water escape slowly. The wood needs to be heated all the way through, ALL the water needs to be turned to steam and allowed to escape. Well, almost all anyway. Once the water is gone, we can up the temp and caramelize the sugar- same deal as the water, all the sugar needs to be caramelized, all the way to the center of the wood.

The resulting thermally modified stick is now drier, little lighter, more stable, little harder, and unappetizing to fungus and bugs. Its also darker, on account of the caramelization. Treated lumber without the poison. Ash is often used and is beginning to hit the market as outdoor decking.

So far I've successfully treated a piece of bass, see the pic, which I will whittle. I don't notice that it's any harder, it certainly is brown- lovely milk chocolate brown, thru & thru. I have a piece of tropical hardwood, looks to be in the rosewood family, part of a pallet I found, I''ve treated that and will cut it open to check for 'doneness' in the center when it cools. For me the advantage will be in using found wood for knife handles- little extra stability, heat treat to kill the screeblies- all in my toaster oven.